1. Field of the Invention
The present invention relates to an ultrasonic ground speedometer utilizing the Doppler effect of ultrasonic waves, which is, for example, adapted for detection of vehicle speed over the ground, and specifically to a speedometer being capable of providing a high accuracy of ground speed measurement over the en&ire ground velocity range to be measured.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various ultrasonic Doppler speed measurement devices. One such conventional ultrasonic Doppler ground speedometer has been disclosed in U.S. Pat. No. 5,054,003 issued on Oct. 1, 1991 (corresponding to Japanese Patent Application No. 1-107319) and U.S. Pat. No. 5,097,453 issued on Mar. 17, 1992 (corresponding to Japanese Patent Application No. 1-107320). As shown in FIG. 1, the above-mentioned conventional ultrasonic Doppler ground speedometer generally includes an oscillator 1 for generating an output signal having a particular wavelength within a wavelength range of 2.6 mm to 3.4 mm essentially corresponding to an oscillator output frequency range of 131 kHz to 100 kHz, an echo sounder transmitter 3 for transmitting an ultrasonic wave having the above noted particular wavelength, and a drive circuit 2 for amplifying the oscillator output signal and for driving the echo sounder transmitter 3 by the amplified signal output therefrom. The transmitter 3 emits an ultrasonic wave 4 with the previously noted particular wavelength in a vehicle forward direction at a predetermined emitting angle against a road surface 5. The conventional ground speedometer also includes an ultrasonic receiver 6 for receiving a reflected ultrasonic wave caused by reflection of the output ultrasonic wave on the road surface 5, and for generating a reflected ultrasonic wave signal. In the prior art ground speedometers, the ultrasonic transmitter 3 and the ultrasonic receiver 6 are both arranged on the same plane. In FIG. 1, reference numeral 7 denotes an amplifier for amplifying the reflected ultrasonic wave signal. The conventional ultrasonic ground speedometer also comprises a signal processor including a multiplier 8 for deriving the frequency difference between the oscillator output signal frequency and the reflected ultrasonic wave signal frequency by multiplying both of the frequencies, a low-pass filter 9 for filtering undesirable noise from the frequency difference signal generated from the multiplier 8, a zero-crossing comparator 10 for waveform-shaping the filtered frequency difference signal representative of the Doppler shift, a pulse counter 11 for counting pulses in the Doppler shift signal from the comparator 10 and for deriving a Doppler frequency, and an arithmetic circuit 12 serving as a Doppler frequency/ground speed convertor for deriving the ground speed on the basis of the output from the pulse counter 11, representative of the Doppler frequency. A frequency controller 13 is also provided for controlling the oscillator output frequency on the basis of the output signal generated from the arithmetic circuit 12 in such a manner as to keep the reflected ultrasonic wave frequency represented by the sum of the output ultrasonic wave frequency and the Doppler shift to a constant value in response to change in the ground speed derived by the arithmetic circuit.
FIG. 2 shows a principle of the ultrasonic Doppler ground speed measurement of the conventional ultrasonic ground speedometer, in which f.sub.o, f.sub.d and F respectively designate an output ultrasonic wave frequency emitted from the transmitter 3, a Doppler shift, and a received ultrasonic wave frequency received by the ultrasonic receiver 6. The received ultrasonic frequency F is equivalent to the sum of the output ultrasonic wave frequency f.sub.o and the Doppler shift f.sub.d. In the previously described conventional ultrasonic ground speedometers, the ground velocity is derived on the assumption that the transmitter 3 and the receiver 6 are both arranged on the same plane and in addition an emitting angle defined by the emitting direction of the output ultrasonic wave and the road surface is equal to a reception angle defined by the reflected direction of the reflected ultrasonic wave and the road surface. As is generally known, supposing that the acoustic velocity represented by c, the arithmetic circuit 12 employed in the conventional ground speedometer derives the ground velocity v from the Doppler frequency f.sub.d according to the following equation. EQU v=cf.sub.d /(2F-f.sub.d)cos.theta.
As appreciated from the above equation, since the emitting angle .theta. is preset to a predetermined angle, the ground velocity v can be calculated by deriving the two values, namely the Doppler shift frequency f.sub.d and the received ultrasonic frequency F.
However, there is a problem that linearity of the calculated ground speed to the actual ground speed is deteriorated at a high speed range, since the calculated ground speed is derived on the assumption that the emitting angle .theta. of the output ultrasonic wave is identical to the reception angle of the reflected ultrasonic wave. That is, since the above assumption is not satisfied at the high vehicle speed range, the difference between the actual vehicle speed and the calculated ground speed tends to become greater according to the increase in velocity at the high speed range in which the vehicle speed substantially approaches to the acoustic velocity.
Referring now to FIG. 3, there is shown a linearity of the calculated ground speed derived by the conventional ultrasonic Doppler ground speedometer with respect to the reference ground speed measured with a spatial filter type ground speedometer. In general, such a spatial filter type ground speedometer can measure the vehicle ground speed with a considerably high measurement accuracy of .+-.0.5 km/h. In the case of test results illustrating the linearity of the calculated ground speed of FIG. 3, the emitting angle .theta. is preset to 45.degree.. As will be appreciated from the graph of FIG. 3, the linearity of the calculated ground speed to the measured reference ground speed is deteriorated at a high speed range of 90 km/h or more. The calculated value of ground speed is 160 km/h at the reference ground speed of 180 km/h. In such an excessively high speed range of 180 km/h, there is an error of approximately 20 km/h, resulting from the previously noted assumption.